Welding is a technology indispensable for producing a structure and, with recent technological advancement, welding can be made for an object made of a material or having a shape for which it has conventionally been difficult to perform the welding. Meanwhile, it is often the case that inspection itself is difficult to perform since a structure produced with an advanced welding technology tends to have a particular groove shape or complicated surface shape. Under such circumstances, importance of an inspection technology for guaranteeing reliability of a welded structure has been increased more than ever before.
As described above, when an inspection for guaranteeing the quality of a welded part is performed for a technically-difficult welding, such as thick plate welding or welding for a structure having a complicated shape, there may be a case where a portion to be inspected enters an ultrasonically blind area or an access itself of an inspection device to the portion to be inspected is difficult, resulting in failure to perform the inspection. Further, assuming that the inspection is performed during or immediately after the welding for shortening work periods, the following problems arise. That is, in the case where the inspection is performed during the welding, the groove shape that is being subjected to the welding may give influence to ultrasonic wave propagation to a region to be inspected, which significantly restricts available inspection methods if the flaw detection surface is small in area. Even in the case where the inspection is performed immediately after the welding, there may be a case where more than half a day is required for reducing heat influence preventing the inspection from being performed immediately. Thus, the time taken until the start of the inspection is wasted.
As a method for solving the above problems, a technique in which welding quality is inspected during the welding operation is proposed in Jpn. Pat. Appln. Laid-Open Publication No. 2001-71139 (Patent Document 1, the entire content of which is incorporated herein by reference). However, this system uses a probe that contacts the surface of an object to be inspected for transmitting ultrasonic waves to or receiving ultrasonic waves from the object, making it difficult to deal with a structure having a narrow portion or complicated surface. Further, the use of the probe requires a contact medium, such as glycerin or water so as to allow the ultrasonic probe to contact the surface of the object to be inspected, complicating post-processing. Further, in the case where the object to be inspected has a high temperature, a special mechanism for preventing damage of the probe is required.
Jpn. Pat. Appln. Laid-Open Publication No. 2007-17298 (Patent Document 3, the entire content of which is incorporated herein by reference) proposes a system in which an ultrasonic wave generation mechanism is attached to a welding mechanism so as to monitor welding operation. However, in this system, it is necessary to directly set the ultrasonic wave generation mechanism in the welding mechanism, which requires modification of an existing welding apparatus and limits an applicable welding method to spot welding or its similar method. Thus, in this system, it is difficult to perform versatile welding, such as butt/groove welding. This is because this system does not directly detect an indication such as reflection echo from an improperly welded part caused in the actual welding, but detects a change in an ultrasonic signal, so that the improperly welded part cannot be identified. Thus, this system is not suitable for repairing a specific part of the welding.
Further, Proceedings of 2010 Spring Conference, pages 63 to 64, The Japanese Society for Non-Destructive Inspection (Non-Patent Document 1, the entire content of which is incorporated herein by reference) suggests a possibility of the inspection using laser ultrasonic waves immediately after and during the welding. However, the technique disclosed in Non-Patent Document 1 is a two-probe method represented by a TOFD (Time of Flight Diffraction) method, etc., in which two probes are disposed astride a welded part and thus cannot deal with a geometric blind area in ultrasonic wave propagation, such as a portion just below the welded part. Further, the irradiation location of reception laser is limited to the surface of a structure, and aperture synthesis processing is employed for an enhancement in the sensitivity, so that only a structure having a planer laser irradiation area larger than a certain size can be targeted.
Further, Patent Document 3 discloses a technique that uses ultrasonic waves other than a surface wave, such as bottom echo, as a reference signal in measurement using the surface wave. However, for an arrangement in which two probes are disposed astride a welded part or for an object to be inspected having whose bottom surface is not flat and smooth, the bottom echo intensity itself serves as a parameter and thus cannot play a role of the reference signal.